The present invention relates to new improved adhesives for binding books and related articles and the production of such adhesives. In particular, the adhesives which are the subject of the present invention relate to hot melt adhesives utilized in both "perfect binding" and "traditional" bookbinding processes.
Although they are distinctly different processes, adhesives are used in both traditional and perfect binding bookbinding. In traditional bookbinding, adhesives are employed in many different steps. In a step referred to as "gluing-off", adhesive is used in addition to thread or wire to bind and seal pages of a book together. In a step referred to as "lining" adhesive is used to attach reinforcing material, such as cloth or paper, to the book spine
In a step referred to as "casing-in", adhesive is used to attach the book's cover to the book spine.
In perfect binding, adhesive alone is used to bind the pages of a book together and attach the book's cover. If only one adhesive application is used to construct a book, it is called a "one-shot process". When two adhesive applications are used, it is called a "two-shot process". In the two shot process, the first application of adhesive binds the pages of the book together. This adhesive is referred to as a primer glue. The second application of adhesive is used to attach the book's cover to the book spine. This adhesive is referred to as a cover glue.
Adhesives used in the above processes fall into two general categories, water-based emulsion adhesives and hot melt adhesives. Water-based emulsion bookbinding adhesives are most often based on vinyl acetate ethylene (VAE) copolymers, but some are based on vinyl acetate (VA) homopolymer. These water-based adhesives are most often used in the traditional process steps of gluing-off and for casing-in edition (hardcover) books. VAE copolymer based water-based adhesives are also used as primer glue in two-shot perfect binding, especially when the cover glue is a hot melt adhesive based on styrenic block copolymers.
Hot melt bookbinding adhesives are most often based on ethylene vinyl acetate (EVA) copolymers. Other hot melt bookbinding adhesive polymers are polyurethanes (PUR) and styrenic block copolymers (SBC). Because they are fast setting, EVA hot melt adhesives are ideal for high speed operations (one-shot) such as perfect binding bookbinding. EVA based hot melt adhesives can be employed as primer and/or cover glue in two-shot perfect binding. As mentioned above, SBC based hot melt adhesives require the use of water-based primer glue to improve its adhesion to the book spine. SBC based hot melt adhesives are often used in the manufacture of rounded spine edition books. PUR based hot melt adhesives are special in that they "cure" with water over time creating a very durable product. PUR hot melts are finding growing use in the manufacture of textbooks.
In hot melt bookbinding adhesives, one function of the polymers mentioned above is to provide strength and flexibility to the adhesive. In this regard, high molecular weight polymers used in hot melt adhesives are often referred to as backbone polymers. These polymers also affect the adhesive's ability to adhere to a given type of paper or other surface. This impact on adhesive performance is greatly influenced by polymer molecular weight and the kind and amount of chemical functionality a polymer possess.
For the above reasons, copolymers of ethylene and unsaturated esters are the most commonly used polymers in bookbinding adhesives. In water-based bookbinding adhesives, VAE copolymers are most often used. In hot melt bookbinding adhesives, EVA copolymers are most frequently employed. In both types of adhesives, ethylene polymer segments impart flexibility while pendant ester (acetate) groups yield an amorphous and chemically functional polymer which are necessary attributes for good adhesion.
Because of their high molecular weight, backbone adhesive polymers cannot be used as the sole component of a hot melt adhesive. Not only are their melt viscosities too high to be useful in bookbinding applications, but their adhesion to paper and other substrates is less than desirable. The most effective way to reduce the melt viscosity of adhesive polymers is with wax(es). These low molecular weight polyethylene homo- and/or copolymers enhance and modify other adhesive properties such as service temperature performance, set-time, cohesive strength, and flexibility. One adhesive property diminished by the addition of wax(es) is adhesion.
Another standard hot melt adhesive ingredient which reduces the melt viscosity of backbone adhesive polymers is tackifier. Although these low molecular weight polymers and resins are not quite as effective as wax(es) in reducing melt viscosity, properly selected tackifiers can do so while simultaneously enhancing both the adhesion and flexibility of hot melt adhesives.
Melt viscosity is but one of many hot melt adhesive properties and/or characteristics that determine if an adhesive is useful in bookbinding. In addition to melt viscosity, important adhesive properties are set-time, cohesive strength, adhesive strength to various substrates, service temperature performance, adhesive flexibility, and adhesive extensibility. However, ultimate performance properties of a book do not solely depend on adhesive performance. The materials and method used in construction of a book are important factors as well.
Important performance properties of books include page-pull, lay-flat, page-flex, cold crack, easy-open, and spine flexibility. Page-pull refers to the force required to pull an individual page from a bound book. Page-pull depends on both the adhesive and surface preparation of the book block. Lay-flat refers to a characteristic of a book to remain open at a given page and how flat it lies (spine up or down). Lay-flat depends on the characteristics of the adhesive and the amount of adhesive applied. Page-flex is a complex function of adhesive characteristics, penetration of the adhesive into the book block, and surface preparation of the book block. Cold crack refers to the temperature at which the spine of a book will crack when it is quickly opened so that its covers touch. Cold crack depends on the adhesive and the amount of adhesive applied. Easy-open refers to the amount of resistance a book offers when it is first opened. Easy-open depends on adhesive characteristics and amount of adhesive applied. Spine flexibility refers to the ability of a book to be opened repeatedly without the spine creasing or wrinkling. Spine flexibility is a function of adhesive properties and amount of adhesive applied.
As with most adhesives, the desired end result is to formulate an adhesive which imparts a good balance of these performance properties. Often bookbinding adhesives are developed to enhance one particular bookbinding property. To be useful, however, these adhesives cannot be too deficient in any one of the performance properties mentioned above.